A diphthong is a combination of two vowel sounds within a single syllable, where the sound glides from one vowel to the other.
However, if you're asking about two identical vowel letters in a word, then that's called gemination. This is where a vowel is doubled within a word, but it's not necessarily pronounced with two distinct sounds.
Here's an example of both:
* Diphthong: The word "coin" has the diphthong "oi", which starts with a "o" sound and glides to an "i" sound.
* Gemination: The word "room" has the vowel letter "o" doubled, but it's pronounced with only one "oo" sound.
Let me know if you have any other questions!